The SOC (system on a chip) does not support any kind of analog component video, including VGA, since the SOC is designed for mobile phone use where this would not be a requirement. Additional components would be needed to generate RGB signals. Additional components would push the price beyond the $25 target and therefore won't happen.

RGB digital

A HDMI/DIV to parallel 3.3v interface PCB, also meant for the Beagleboard:

The schematics for apples iPhone 3gs and 4g suggest they speak DSI, thus they can probably be connected directly. The older iPhones use a "Mobile Pixel Link" connection from National Semiconductor. The 3GS panel (480×320) goes as low as US $14.88, while the 4G one (960×640, possibly the LG LH350WS1-SD01, with specifications) can be had for US $17.99 or as low as US $14.28. The connectors used might be an issue, but this connector might fit. Additional circuitry might be necessary to provide the display with required 1.8V and 5.7V for operation, and an even higher voltage for the backlight.

The Raspberry Pi provides one clock lane and two data lanes on the S2 connector, as can be read from the schematics. It is currently unknown whether this is enough to drive the iPhone 4G screen, as that screen seems be driven with three data lanes in its original application.